Metallic Hollow Sphere Structures Manufacturing Process
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Metallic Hollow Sphere Structures Manufacturing Process C. Davoine, A.Götzfried, S. Mercier, F. Popoff, A. Rafray, M. Thomas, V. Marcadon ONERA, BP72 - 29, avenue de la Division Leclerc, 92322 Châtillon, France
ABSTRACT This paper focuses on manufacturing process of regular Metallic Hollow Sphere Structures (MHSS) through brazing technique. As a large stress level is generally confined into the necks formed by brazed spheres, the influence of the filler material on mechanical behavior of cellular metal has been studied. The microstructures of joints resulting from nickel hollow spheres brazing with different commercial fillers “MBF 30” and “MBF 1006” were compared by Scanning Electron Microscopy (SEM) and microhardness testing. These studies revealed a wide boron diffusion into nickel shells through grain boundaries for “MBF 30” brazing, with the formation of borides in a fine brittle eutectic structure. Conversely it was observed that the eutectic structure concentrates at the necks for “MBF 1006” and can be completely eliminated by diffusion-brazing, despite of the shells thinness. The uniaxial compressive tests of HSP specimens have shown two different strain mechanisms depending on brazing process. INTRODUCTION Reducing material weight is a permanent challenge for industry, especially for aerospace applications. With this objective, innovative lightweight construction materials were devised in recent years, and cellular metallic materials seem to be a very promising material class [1]. Moreover, cellular metal structures has been gaining interest for multi-functional applications where acoustic or mechanical damping and high temperature application is required. Compared to porous metal which contains a multitude of pores, a cellular metal is a material divided into distinct cells [2]. Honeycombs or MHSS belong to this class of materials. In this paper, we focused on MHSS, but the processes are also studied to devise some other kinds of cellular metal, especially regular array of metallic tubes. The main advantage of this class of material, beside the regularity of its structure which should ensure a good reproducibility of mechanical properties, is to be used in a material-by-design approach [3]. Indeed the thickness and diameter of the cells, the nature of the constitutive metal, the pattern for the piling up can be varied. This allows to design the cellular structure in order to reach the target properties imposed on the material by design requirements. Hollow spheres can be produced through galvanic methods or through a powder metallurgy based manufacturing process. In both cases, EPS (expanded poly styrol) spheres are used to obtain the spherical shape before being eliminated through pyrolyse or through chemical dissolution. Various joining technologies such as brazing and adhering can be used to assemble the single hollow spheres together. An advantage of brazing hollow spheres is to create a neck, whose mechanical behavior and morphology could be used as a further design parameter for the op
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